A known type of the all terrain vehicle includes a motor-driven power steering apparatus disposed between an input shaft connected to an operating handlebar and an output shaft connected to a wheel side. See, for example, Japanese Utility Model Laid-open No. Hei 3-1877.
FIGS. 1 and 6 of Japanese Utility Model Laid-open No. Hei 3-1877 show an arrangement wherein an input shaft 5 is installed on a lower portion of an operating handlebar 3 with an output shaft 11 being connected to the input shaft 5 via a motor-driven power steering apparatus. The motor-driven power steering apparatus includes a torsion bar 19 disposed between the input shaft 5 and the output shaft 11. In addition, a stopper mechanism 101 is disposed on the side of the input shaft 5. The stopper mechanism 101 includes a lock member 103 that projects from the input shaft 5 and a pair of stopper members 105 projecting from a chassis 37. The stopper mechanism 101 restricts the maximum steering angle of the operating handlebar 3.
The motor-driven power steering apparatus includes a steering torque sensor 7 that includes the torsion bar 19, a reduction gear mechanism 9, a drive motor 17, a control unit 15 shown in FIG. 3 of Japanese Utility Model Laid-open No. Hei 3-1877 and the like.
The input shaft 5 is rotated with the operating handlebar 3 to twist the torsion bar 19. This produces a relative rotational angle in the input shaft 5 and the output shaft 11. A potentiometer detects this relative rotational angle and a signal corresponding to the detected relative rotational angle is applied to the control unit. Based on the signal, the control unit 15 controls the drive motor 17 so as to give the side of the output shaft 11 a driving force corresponding to a steering torque detected by the steering torque sensor 7.
FIGS. 5 and 8 of Japanese Utility Model Laid-open No. Hei 3-1877 illustrate a stopper mechanism 33 on the side of the output shaft 11. The stopper mechanism 33 includes a lock member 35 projecting from the output shaft 11 and a pair of stopper members 39 mounted on the chassis 37. The stopper mechanism 33 restricts a range of rotational angles of the output shaft 11.
Assuming that the motor-driven power steering apparatus includes only the stopper mechanism 101 on the side of the input shaft 5 as shown in FIG. 1 of Japanese Utility Model Laid-open No. Hei 3-1877. Further assuming a condition, in which the operating handlebar 3 is turned until the lock member 103 contacts the stopper member 105 of the chassis 37. Even in this condition, the wheels of a vehicle can be forcibly turned clockwise or counterclockwise by bumps and potholes in a road surface. As the wheels are turned clockwise and counterclockwise, the output shaft 11 can be rotated through a rotational angle greater than the rotational angle of the input shaft 5. This produces a relative rotational angle in the input shaft 5 and the output shaft 11, thus operating the drive motor 17 and increasing power consumption of the drive motor 17.
Assuming, on the other hand, that the motor-driven power steering apparatus includes only the stopper mechanism 33 on the side of the output shaft 11 as shown in FIGS. 5 and 8 of Japanese Utility Model Laid-open No. Hei 3-1877. Further assuming a condition, in which the wheels of the vehicle are forcibly turned clockwise or counterclockwise by bumps and potholes in the road surface, causing the stopper member 39 of the stopper mechanism 33 to abut on the lock member 35. Even in this condition, as the wheels are turned clockwise and counterclockwise, the input shaft 5 can be rotated through a rotational angle greater than the rotational angle of the output shaft 11. This produces a relative rotational angle in the input shaft 5 and the output shaft 11, thus operating the drive motor 17 and again increasing the power consumption of the drive motor 17.